Related arts known as those in this technical filed are disclosed in Japanese Patent Application Publications Nos. 2009-238633 and 2008-27874. An LED lighting device described in Japanese Patent Application Publication No. 2009-238633 prevents a rise of the output voltage by flowing a standby current while the LED is off. The standby current is flowed by driving a switching power supply at a pre-lighting duty ratio set to obtain a voltage lower than an output voltage applied when the LED is turned on and also by turning on a bypass circuit serving as a dummy load connected in parallel with the LED.
A light irradiation system described in Japanese Patent Application Publication No. 2008-27874 includes a driving unit configured to supply an operating current based on an instruction value of an LED operating current in an ON period when the LED is lighted, and actively supply a standby current that is lower than the operating current and is based on an instruction value of the standby current, before the ON period starts while the LED is off.
Arts described in Japanese Patent Application Publications Nos. 2009-238633 and 2008-27874 prevent a drop of the output voltage by flowing a predetermined standby current all the time when the LED is off, and also prevent a temporary shortage of LED illumination intensity that would otherwise occur because the LED operating current does not reach a given value during a rise time of the output voltage due to a delay of the output voltage rise in the rise time for turning on the LED.
However, the flowing of the predetermined standby current while the LED is off leads to a waste of energy.
Further, according to Japanese Patent Application Publication No. 2009-238633, the output voltage in the standby mode is reduced by switching the output current in the lighting mode to the standby current in the standby mode. That is, use of a feedback control signal for reducing the output voltage in the standby mode is not enough to prevent power supply to an output from a transformer by a switching operation, and thereby causing the output voltage to rise slightly. The standby current is a load current for consuming the power supplied by the switching operation. Also requiring a standby current, the technique described in Japanese Patent Application Publication No. 2008-27874 is considered to use a converter similar to the one described in Japanese Patent Application Publication No. 2009-238633.
An object of the present invention is to provide a constant current power supply device capable of controlling an output voltage at a setup voltage lower than an operating voltage without flowing a standby current.